JPH0634406B2 - Intrinsic semiconductor electro-optical device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The wavelength range of 3-5 μm and 8-12 μm (with the lowest absorption by the atmosphere) has several applications, including military applications related to infrared imaging. This is
It has motivated significant research efforts to develop intrinsic semiconductor detectors that can operate at these wavelengths. Intrinsic detectors are such that their absorption is based on the excitation of carriers from the valence band to the conduction band. Although many materials appear to be suitable for making imaging arrays for the 3-5 μm region, any bulk morphology III-
Group V materials also do not have sufficiently small bandgap values for cooled (eg 77 ° K) (low noise) detector operation in the critical 8-12 μm region. As a result, it was necessary to consider a large number of Group II-VI and Group IV-VI substances as potential candidate substances for 8-12 μm applications.

Most of the work for these applications has recently focused on the Group II-VI alloys, Hg _1-x Cd _x Te. This alloy is E
g is ~ 1.6 eV (x = 1.0) to 0.0 eV (x ~ 0.16
) With a bandgap (at 0 ° K). And alloys with x = 0.205 have the necessary bandgap for 77 ° K detector operation in the 8-12 μm atmospheric window (eg Semiconductors and Semim).
et al, Willardson & Beer, Ed, Vol. 18,
"Mercury Cadmium Telluride", Academic Pr
See ess (1981). However, Hg containing a large amount of Hg
CdTe alloys have a number of metallurgical and equipment-related problems, so that a satisfactory 8-12 μm of these materials, if not achievable, is obtained.
It makes the use of imaging arrays difficult. Some of these issues are listed below.

 1) Hg Cd Te is mechanically brittle.

2) It is difficult to control the hole concentration in p-type Hg Cd Te. N-type "doping" is typically un-annealed implantation dameg
made by e).

3) Above 100 ° C, Hg outdiffusion becomes a problem (the n-type substance is converted to p-type). This is
It suggests long-term stability issues and eliminates the possibility of high temperature device processing of Hg Cd Te wafers.

4) The bandgap of Hg _1-x Cd _x Te and its associated cutoff wavelength for IR detectors change rapidly with alloy composition.
As a result, to avoid large lateral non-uniformities in detector performance (poor device yield), the composition must be kept within ± 0.3% over the entire wafer.

5) The extremely small electron effective mass (me ^* ) in Hg-rich alloys makes the tunneling phenomenon from the conduction band to the valence band important. This gives rise to experimentally observed noise components that cannot be reduced by cooling the detector. Moreover, the tunnel phenomenon is
▼ ⁻¹ ₀ ▲ ≡ (dI / dV) | Zero bias defined by _{v = 0} (reduces the photodiode operating resistance at R _0. This means that the charge coupled device / (CCD) readout has a constant minimum R _The need for ₀ prevents it from working well in the case of an 8-12 μm detector array that utilizes the readout of a charge coupled device.

Accordingly, it is an object of the present invention to provide a new electro-optical device that can operate in new wavelength regions (e.g. 8-12 [mu] m) not previously available with the special semiconductor materials used in the device.

Another object of the present invention is to provide a new device that overcomes or ameliorates the problems of the prior art, including those noted above for Hg Cd Te devices.

Another object of the present invention is the strained layer sup
erlattice (SLS) to achieve these effects.

Upon further study of the specification and claims, other objects and advantages of the invention will be apparent to those skilled in the art.

The present invention is an intrinsic semiconductor electro-optical device consisting of a pn junction that responds to electromagnetic radiation of wavelength 8-12 μm when cooled and consists essentially of a III-V semiconductor material: The reflective reactive pn junction consists of a strained layer superlattice (SLS) with alternating layers of two different III-V semiconductors; the lattice constants of the two semiconductors in bulk form are mismatched. The total strain is exerted on each pair of alternating semiconductor layers in the SLS structure, and the proportion of the total strain acting on each layer of the pair is inversely proportional to the ratio of the layer thickness of each layer of the pair. The first set of alternating III-V semiconductor layers is In As _1-x Sb _x , where x is approximately
0.5-0.7), and the second set of alternating layers comprises a III-V semiconductor having a lattice constant greater than that of the layers in the first set; lattice constant mismatch And the strain due to the layer thickness ratio are effective in narrowing the band gap of the first set of III-V layers, thereby increasing the intrinsic radiation absorption characteristics of the layers during cooling, The above objects have been achieved by providing a semiconductor electro-optical device characterized in that it is modified to contain wavelengths in the 8-12 μm region which are longer than the wavelengths to which the individual layers respond when cooled and in bulk form. Of.

The invention will be better understood with reference to the accompanying drawings, in which other objects, features and attendant advantages of the invention will be more fully apparent.

An SLS system is a series of thin layers of at least two different semiconductor materials, which semiconductor materials have different lattice constants in bulk form. SLS
This lattice mismatch in the system
The effect of is to align two adjacent lattices under the strain introduced by the forced alignment. Under well-known conditions such as maximal mismatch (lattice constants differing by about 7-8%) and reasonably thin layer thickness (eg generally less than about 500Å) Lattice defects, such as misfit dislocations, are suppressed as energetically inappropriate. In addition, the structure is based on the underlying layers, for example graded lay.
ers) to act as a barrier to dislocation propagation. As a result, extremely high crystal lattice integrity is obtained for each layer. Throughout the two adjacent layers will be in bulk morphology than would otherwise be possible. Therefore, SLS structures are often employed when it is desired to grow adjacent layers with different lattice constants. As used herein, the term "SLS" means a structure having the necessary layer properties to avoid and / or minimize misfit dislocations as described above.

Intrinsic strains in SLS systems are necessary to suppress unwanted lattice defects, but also disturb other properties of the system. For example, it is known that this strain causes a change in the effective band gap of each layer. That is, the effect of one layer on the adjacent layer results in an increase in the bandgap, while the effect of the adjacent layer on the first layer results in a decrease in the bandgap. When applied by the prior art, the effect was an unavoidable characteristic. It has now been found that these effects can be advantageous in electro-optical devices that allow the semiconductor material to respond to wavelengths that the semiconductor material does not respond to in bulk form.

Usually, Group III-V semiconductor alloys have a significant 8-1 degree at low temperatures such as 77 ° K, which is required for high S / N ratios.
It cannot be operated in the wavelength range of 2 μm. For example, in FIG. 1, λ _c (μm), that is, the cutoff wavelength λ _c ≡1.24 / Eg of the bulk In As _1-x Sb _x at 77 ° K.
The dependence of (where Eg is a band gap) on x is shown. Obviously, when x is about 0.61, the maximum response wavelength is slightly longer than 9.0 μm, but 12 μm.
Is insufficient for the intended use up to.

With alternating layers of two different InAsSb alloys combined in the SLS, the overall response wavelength of the SLS is extended to longer wavelengths as desired. This is accomplished by the proper choice of the two alloys so that the strain due to the mismatch for one alloy reduces its bandgap. As a result, the SLS is then used as an intrinsic detector of longer wavelength radiation corresponding to the narrower gap. In general, a mismatch between one layer and another with a larger lattice constant tends to stretch the normal lattice arrangement of the first layer in a plane parallel to the interface,
As a result, the band gap is reduced. Mismatches with layers having a smaller lattice constant than the layer cause compression effects in the plane parallel to the interface, increasing the bandgap and consequently decreasing the cutoff wavelength. These effects are known per se and the magnitude of the bandgap change is expressed as a function of the degree of mismatch. Therefore, using conventional knowledge,
The degree of mismatch required to change the bandgap to the desired degree can be calculated easily and reliably. The exact degree of mismatch and the change in bandgap can be routinely determined according to conventional experimental procedures based on the initial estimates.

In general, within a Group III-V direct-gap compound semiconductor for a given lattice mismatch, the bandgap changes are about the same for the previous members. Therefore, the largest relative change in response occurs for semiconductor materials with a relatively small bandgap in the bulk. For this reason, the InAsSb system is preferred. It has a relatively small bandgap in the bulk III-
It is a Group V substance. However, it goes without saying that the present invention is also applicable to materials having a relatively large band gap in bulk.

The next consideration is the essential constraint on the degree of mismatch allowed within the range of effective SLS. Usually, in mismatches that differ by more than 7-8% in lattice constants, unacceptable crystal defects, such as dislocations across the layers, occur due to large strains. The mismatch must be small enough to obtain a valid SLS system.

After determining the magnitude of the lattice mismatch required to give the desired bandgap change ΔEg, the layer thickness ratio also needs to be considered. Of course, the effect of lattice mismatch is to strain each of the two layers involved, acting to narrow the gap on one side and widen the other. The degree of each of these individual effects is determined by the ratio of the two layer thicknesses and the elastic constants of the two layers according to the well-known functional dependence (eg Osbourn,
J. Vac. Technol. Bl (2), April-June 1983). Thus, for a material with a layer thickness ratio of 1/1 and essentially the same elastic constant, equal strains due to lattice constant mismatch will act on each of the two layers. When the layer thickness ratio is 2/1, the thin layer has about twice as much strain as the thick layer. That is, the layer thickness ratio is a design parameter that gives another degree of freedom for obtaining an appropriate strain level in achieving a desired ΔEg. The layer thickness ratio, along with the mismatch itself, facilitates fine tuning the device design. In a typical case, the layer thickness ratio is within the range of 10/1 to 1/10.

Once the lattice constant mismatch and the layer thickness ratio have been chosen, or, preferably, a combination of the two, the actual layer thickness is again determined using the guidelines and conventions described herein. To be selected. Viewed solely from the effects of SLS, it is only necessary that the induced strain be thin enough to suppress misfit dislocations. In the typical case, the layer thickness is in the range of 30 to 500Å or 50 to 500Å. And the lower limit is mainly determined by the growth technology.

However, the SLS effect is not the only subject for consideration. S
It is known that certain quantum mechanical effects occur when the layer thickness in LS is small (see, for example, Osbourn, supra). In fact, a very convenient photovoltaic device has been proposed based on the SLS structure and this quantum effect (see, for example, US patent application No. 512059 filed July 8, 1983). A full explanation of quantum mechanical phenomena is given in the above references, so only a brief summary of its effects will be given here in order to clarify the problem.

In essence, alternating layers of different semiconductor materials cause quantum-mechanical perturbations of the bandgap energy levels beyond the effect of strain due to mismatching. As a result, the overall structure will have an effective bandgap that will grow when this quantum mechanical effect is more effective. This is due to the formation of quantum wells that have been characterized as being caused by the high potential barrier formed by the higher bandgap alternating material surrounding the lower bandgap alternating layers.

As can be clearly seen, under the right circumstances, the quantum well effect can counter the desired effect based on the mismatch strain. For example, in the group III-V system described above, it is desirable to narrow the bandgap of the material with the lowest bulk bandgap in order to provide responsiveness to longer wavelengths. The SLS described above has alternating layers of low and high bandgap materials. The net effect of mismatch strain is to further reduce the bandgap of the lowest material. However, as already explained, the quantum mechanical effect increases the effective SLS bandgap.

Advantageously, with proper choice of the absolute thickness of the SLS layer, the quantum well effect can be suppressed to such an extent that the mismatch effect dominates. This point is fully explained in the aforementioned US patent application No. 512059 (in particular FIG. 1). As the thickness of the "quantum well layer," or lower bandgap layer, is increased, the quantum well energy decreases sharply. That is, the system is closer to the bulk state and only the lowest quantum well levels are occupied. Conveniently, the thickness that minimizes the quantum well effect is within a range where the SLS effect can still operate, for example about 100 Å or more or 200 Å or more.

Thus, if the quantum well effect opposes the effect obtained with strain due to mismatch, the film thickness will be somewhat longer than the layer thickness otherwise. In general, when trying to minimize the quantum well effect, the highest thickness still compatible with the achievement of SLS systems is adopted. That is, it is the maximum thickness that can be employed at a given strain for a given composition.

As can be clearly seen, the strain, layer-thickness ratio, layer-thickness per se, which is due to inconsistencies, becomes apparent once the basic inventive concept of the device is made clear by the explanation given here and the readily available common knowledge. , The details of the semiconductor composition, etc., corresponding to the desired lattice constant are easily calculated and determined, which gives a system with a response different from that of each individual semiconductor material in bulk. it can. These a-priori calculations give a very reliable indication, so routine and routine parametric experiments (eg, conventional ones) can be used to determine and optimize the structural details of the device in a given case. Only a semiconductor measuring system is used). The functional dependencies and basic data for performing the above calculations are well known and they are routinely applicable to the present invention. For example, O
sbourn, J.M. Vac. SCi. Technol. Bl (2), April-
June 1983, p.379; Gourley et al., Ibid., P.383; Sem
iconductors and Semimetals, Willardson et al., Vol.
4, "Pysics of III-V Compounds," Academic
Press (1968), all pages, especially 327-330; Dingle,
Crit. Rev. Solid State Sci., Vol. 5, p.585
(1975); Esaki et al., Crit. Rev. Solid Stat
e, Vol. 6 (1976); B. Panish, Science,
Vol. 208, p.916 (1980); Madhukar, J.M. V
ac. Sci. Technol., Vol. 20, p.419 (1982);
G. C. Osborn J. Appl. Phys., Vol. 53, p.1
586 (1982); C. Osplarn et al., Appl.Phys.Let
t., Vol. 41, p.172 (1982); W. Matthew S
J. et al. Cryst. Growth, Vol. 27, p. 118 (197
4); W. Matthews et al., Id., Vol. 29, p. 273
(1975); W. Matthews et al., Id., Vol. Three
2, p.265 (1976); C. Osbourn, J.M. Vac. S
ci. Technol., Vol. 21, p.469 (1982); Din
Gle et al., Phys. Rev. Lett., Vol. 33, p. 827
(1974); R. White et al., Phys. Rev. Lett.,
Vol. 47, p.879 (1981); Bastard, Phys.
Rev., Vol. B25, p. 7584 (1982); J. Fritz
Et al. Solid State Commun., Vol. 45, p. 323
(1983); J. Fritz et al., “Proceedings of 1982
Int. Symposium on Ga As and Related Compo
unds ”, (Institute of Physics, Berkshire, 19
83), p. 241; H. Dohler, Jr. Vac. S
ci. Technol., Vol. 16, p. 851 (1979).

The second layer used with the first main layer is generally not a determinant. The main requirements are that the second layer have the necessary lattice constants to provide the required mismatch strain and that the second layer provides the SLS structure when grown with the main layer to the required thickness. . For example, as described below, both the second layer and the first layer are made from the In As Sb system. However, there are other semiconductor layers with the required lattice parameters. Most of them are of Group III-V.

From what has already been mentioned, it can be seen that both layers in the SLS change the bandgap energy. The system is designed primarily to take into account certain changes to one set of layers. Under most circumstances, changes in other sets of layers are well matched to the intended operation of the device, without any hindrance. For example, for Group III-V systems where longer cutoff wavelengths are required, it is not critical that the second set of alternating layers can reduce the cutoff wavelength. Because the whole system responds at longer wavelengths, which is the main purpose. However, under the condition where the two effects can produce opposite effects, the above-mentioned parameter of the layer thickness ratio is
It can be used to maximize the magnitude of the desired effect and minimize the magnitude of the undesired effect.

The manufacturing of devices is quite routine and general,
The details are described herein. Any conventional method can be used to grow the semiconductor layer to the required thickness. Although epitaxy can be used, the most preferred is molecular beam epitaxy (see, eg, US Patent Application No. 512059, supra).

In general, the dimensions of the device of the present invention and the number of layers used in the SLS structure are primarily concerned with the expected intensity of the incident, as well as having the device capable of absorbing all incident electromagnetic waves, thereby increasing the sensitivity of the device. It is routinely decided after considering the demand to raise.

Due to its desirable properties, as already mentioned, the device according to the invention preferably consists of a III-V semiconductor material. More preferably, relatively low bandgap materials are used. This is because the effect of increasing the cutoff wavelength is relatively large. In this regard, In Ga A
s Sb is preferred. Most preferred is In As Sb.
In these systems, a 40-50% bandgap size change is achieved due to the relatively small bandgap (eg, 0.1-0.2 eV). Figure 2 shows In As
_The cut-off wavelength achievable at 77 ° K for systems based on strained bulk In As _0.39 Sb _0.61 layers in SLS of _0.39 Sb _0.61 / In As _1-x Sb _x is shown for x> 0.61. It is a thing. The value of x = 0.61 is based on the fact that this composition has the lowest bulk bandgap shown in FIG. Therefore, to achieve a 12 μm response, less strain is needed than with the other compositions shown in FIG.

As is clear from Fig. 2, when the value of x is about 0.73,
The cutoff wavelength reaches a value of 12 μm as desired. At values above 0.73, the bandgap of the In As _0.39 Sb _0.61 layer decreases to values below 0.1 eV. Figure 2 shows x
= .73, that is, λ _c = 12 μm, two SLs
It is based on the fact that the lattice mismatch of the S layer is 0.86%. The data in Fig. 2 is for a layer thickness ratio of 1.0. Since it is necessary to minimize the quantum well effect for the reason described above, in the case of the In As _0.27 Sb _0.73 layer, the actual layer thickness is 100 to 300 Å. Also, if it is desired to use a layer thickness ratio of 1.0, the actual layer thickness is in the same range for In As _0.39 Sb _0.61 . In provided by the second layer (x = 0.73) to obtain a detector response of 12 μm.
The amount of strain in the As _0.39 Sb _0.61 layer is Δα / α to 0.4%. This is well within the strain range experimentally adopted in the two SLS material systems described above. For example, G.I.
C. Osbourn et al., Appl. Phys. Lett., Vol. Four
1, p.172 (1982); J. Fritz et al., “Galli
um Arsenide and Related Compounds ”, 1982,
Inst. Phys. Conf. Ser., Vol. 65, p. 241
See (1983).

Using the above as a guide, the details can be routinely determined similarly in the case of the InGaSb system.

The novel device of the present invention has many advantages in addition to the basic feature of providing wavelength response in regions previously unavailable with certain materials. That is, the group III-V InAsSb SLS material of the present invention is
In addition to extending the cutoff wavelength of materials such as In As Sb alloys to the region of interest, the above-mentioned Hg Cd T
Eliminate various problems in case of e. Especially problem 1) ~
3) is easily avoided because it uses a Group III-V substance. Further, as shown in FIG. 2, SLS of In As Sb
Problem 4) is solved because the cut-off wavelength at 2 depends only slowly on the composition (dEg / dx <4meV /%). Also, due to this slow dependence, the compositional change on the side of 1.5% that can be easily achieved is
s Sb is acceptable through SLS wafer cross section.
Problem 5) is solved based on the inherent ability of these devices to increase the effective mass of superlattice carriers along the direction perpendicular to the superlattice layer. It is generally known that this increase affects the normal superlattice migration mechanism in the vertical direction, but recently this increase tunnels the conduction band into the valence band in the reverse-biased superlattice diode structure. (Eg, D. L. Smith et al., Appl. Phys. Lett., V.
ol. 43, p. 180 (1983)). This property provides a strong incentive to use superlattice materials rather than bulk materials for device applications that require very small bandgaps.

The device of the present invention may be used in all configurations where prior art analog devices have been used, such as pn junction based intrinsic detectors. One such device is shown in FIG. It is In Sb (eg p-
It consists of an ordinary substrate such as (type). Under conditions where the lattice constant of the SLS layer is sufficiently different from the lattice constant of the substrate that there is a significant likelihood that unacceptable lattice defects will occur, the SLS structure itself and this substrate will be In addition, as shown in FIG. 3, it is possible to interpose a grade layer. For example, In As _0.39 Sb _0.61
For SLS of / In As _0.27 Sb _0.73 , the preferred grade layer is In As _1-x Sb _x , where x is 1.0
.About.0.67), and the n-type is used in the arrangement of FIG. If the lattice constant of the SLS is fairly closely matched to that of the substrate, then the grade layer can be omitted, as is well known in conventional devices. Of course, n-
The n-n-p structure is only one of many possible configurations.
And this is likewise familiar to the person skilled in the art.

In general, the devices of the present invention are made to operate at temperatures as low as 77 ° K to provide maximum signal-to-noise ratio by reducing noise. However, devices operating at room temperature are well within the scope of the invention.
Similarly, two or more SLS systems of the present invention may be used to make a multi-junction device, if this is convenient for a given application. Conventional tunnel junctions are used as intermediate shorting interconnects when the additive effects of various junctions are desired. However,
It is also possible to use the two SLS system of the present invention if it is desired to isolate a relatively narrow portion of the increased response range. This is achieved by stacking the two SLS systems of the present invention. The top system absorbs unwanted radiation and only radiation of the desired wavelength reaches the underlying SLS. Only the latter SLS receives normal electrical contact. Therefore, the current will be proportional only to its response.

From the foregoing description, those skilled in the art can easily understand the essential features of the present invention, and also use the present invention in various applications and conditions without departing from the scope of the claims of the present invention. Therefore, various changes and modifications can be made to the present invention.

[Brief description of drawings]

FIG. 1 is a graph showing the cutoff wavelength of bulk In As _1-x Sb _x at 77 ° K as a relation of x. Figure 2 shows In As _0.39 Sb _0.61 / In As _1-x Sb _x SLS.
Medium distorted bulk In As _0.39 Sb _0.61 layer 77 ° K
3 is a graph showing the cutoff wavelength in FIG. 3 as a function of x (> 0.61). FIG. 3 is an explanatory view showing one constitution of the device of the present invention.

Claims (11)

[Claims] 1. A strained layer superlattice (SLS) having alternating layers of two different III-V semiconductors, wherein the lattice constants of the two semiconductors in bulk form are mismatched. The proportion of the total strain exerted on each layer of each pair of said alternating layers is inversely proportional to the ratio of the layer thicknesses of each layer of said pair, the first set of said alternating layers being InAs _1-x Sb _x (where X Is between 0.5 and 0.7) and the second set of alternating layers has a lattice constant greater than the lattice constant of the layers in the first set.
Intrinsic semiconductor electro-optical device having a pn junction, characterized in that it comprises a group III semiconductor and strain caused in each layer of each pair of said alternating layers due to lattice constant mismatch narrows the band cap of said first set. . 2. The intrinsic semiconductor electro-optical device according to claim 1, wherein the pn junction is responsive to electromagnetic radiation with a wavelength of 12 μm. 3. A second set of alternating layers InAs _1-y Sb y intrinsic semiconductor electrical described in paragraph 1 claims _(wherein Y is the at is 0.65 or more), characterized in that it consists of Optical device. 4. Mezzanine that the second set of alternating layers consists of InAs _1-y Sb _{y (wherein} Y is 0.73 or higher), and X in said first set of layers is 0.61 The intrinsic semiconductor electro-optical device according to claim 2. 5. The layer thickness in the SLS is 30 to 500.
The intrinsic semiconductor electro-optical device according to claim 3, which is in the range of angstrom. 6. The layer thickness in the SLS is 50 to 500.
The intrinsic semiconductor electro-optical device according to claim 4, which is in the range of angstrom. 7. The layer thickness of each set is such that the quantum well effect is said SL.
Small enough to occur in a layer of S, the quantum well effect increases the bandgap of the first set of layers, thereby partially canceling the effects of strain due to the lattice mismatch. And the first set of layer thicknesses is chosen to be large enough to minimize the quantum well effect, while being small enough to form SLS at the layer thickness ratio. The intrinsic semiconductor electro-optical device according to claim 4, characterized in that 8. The layer thickness in the SLS is 30 to 500.
The intrinsic semiconductor electro-optical device according to claim 7, which is in the range of Angstrom. 9. The layer thickness in the SLS is 50 to 500.
The intrinsic semiconductor electro-optical device according to claim 7, which is in the range of Angstrom. 10. The intrinsic semiconductor electro-optical device according to claim 2, wherein the response at 12 μm occurs at a device temperature of 77 ° K. 11. The intrinsic semiconductor electro-optical device according to claim 9, wherein a ratio of layer thickness in the SLS is 1.0.